A mutation in the amino terminus of a hybrid TrpC-TonB protein relieves overproduction lethality and results in cytoplasmic accumulation

The Intrinsically Disordered Region of ExbD Is Required for Signal Transduction

The TonB system actively transports vital nutrients across the unenergized outer membranes of the majority of Gram-negative bacteria. In this system, integral membrane proteins ExbB, ExbD, and TonB work together to transduce the proton motive force (PMF) of the inner membrane to customized active transporters in the outer membrane by direct and cyclic binding of TonB to the transporters. A PMF-dependent TonB-ExbD interaction is prevented by 10-residue deletions within a periplasmic disordered domain of ExbD adjacent to the cytoplasmic membrane. Here, we explored the function of the ExbD disordered domain in more detail. In vivo photo-cross-linking through sequential pBpa substitutions in the ExbD disordered domain captured five different ExbD complexes, some of which had been previously detected using in vivo formaldehyde cross-linking, a technique that lacks the residue-specific information that can be achieved through photo-cross-linking: two ExbB-ExbD heterodimers (one of which had not been detected previously), previously detected ExbD homodimers, previously detected PMF-dependent ExbD-TonB heterodimers, and for the first time, a predicted, ExbD-TonB PMF-independent interaction. The fact that multiple complexes were captured by the same pBpa substitution indicated the dynamic nature of ExbD interactions as the energy transduction cycle proceeded in vivo In this study, we also discovered that a conserved motif-V45, V47, L49, and P50-within the disordered domain was required for signal transduction to TonB and to the C-terminal domain of ExbD and was the source of motif essentiality.IMPORTANCE The TonB system is a virulence factor for Gram-negative pathogens. The mechanism by which cytoplasmic membrane proteins of the TonB system transduce an electrochemical gradient into mechanical energy is a long-standing mystery. TonB, ExbB, and ExbD primary amino acid sequences are characterized by regions of predicted intrinsic disorder, consistent with a proposed multiplicity of protein-protein contacts as TonB proceeds through an energy transduction cycle, a complex process that has yet to be recapitulated in vitro This study validates a region of intrinsic disorder near the ExbD transmembrane domain and identifies an essential conserved motif embedded within it that transduces signals to distal regions of ExbD suggested to configure TonB for productive interaction with outer membrane transporters.

Mutations in Escherichia coli ExbB transmembrane domains identify scaffolding and signal transduction functions and exclude participation in a proton pathway

The TonB system couples cytoplasmic membrane proton motive force (pmf) to active transport of diverse nutrients across the outer membrane. Current data suggest that cytoplasmic membrane proteins ExbB and ExbD harness pmf energy. Transmembrane domain (TMD) interactions between TonB and ExbD allow the ExbD C terminus to modulate conformational rearrangements of the periplasmic TonB C terminus in vivo. These conformational changes somehow allow energization of high-affinity TonB-gated transporters by direct interaction with TonB. While ExbB is essential for energy transduction, its role is not well understood. ExbB has N-terminus-out, C-terminus-in topology with three TMDs. TMDs 1 and 2 are punctuated by a cytoplasmic loop, with the C-terminal tail also occupying the cytoplasm. We tested the hypothesis that ExbB TMD residues play roles in proton translocation. Reassessment of TMD boundaries based on hydrophobic character and residue conservation among distantly related ExbB proteins brought earlier widely divergent predictions into congruence. All TMD residues with potentially function-specific side chains (Lys, Cys, Ser, Thr, Tyr, Glu, and Asn) and residues with probable structure-specific side chains (Trp, Gly, and Pro) were substituted with Ala and evaluated in multiple assays. While all three TMDs were essential, they had different roles: TMD1 was a region through which ExbB interacted with the TonB TMD. TMD2 and TMD3, the most conserved among the ExbB/TolQ/MotA/PomA family, played roles in signal transduction between cytoplasm and periplasm and the transition from ExbB homodimers to homotetramers. Consideration of combined data excludes ExbB TMD residues from direct participation in a proton pathway.

Taking the Escherichia coli TonB transmembrane domain "offline"? Nonprotonatable Asn substitutes fully for TonB His20

The TonB system of Gram-negative bacteria uses the proton motive force (PMF) of the cytoplasmic membrane to energize active transport of nutrients across the outer membrane. The single transmembrane domain (TMD) anchor of TonB, the energy transducer, is essential. Within that TMD, His20 is the only TMD residue that is unable to withstand alanine replacement without a loss of activity. H20 is required for a PMF-dependent conformational change, suggesting that the importance of H20 lies in its ability to be reversibly protonated and deprotonated. Here all possible residues were substituted at position 20 (H20X substitutions). The His residue was also relocated throughout the TonB TMD. Surprisingly, Asn, a structurally similar but nonprotonatable residue, supported full activity at position 20; H20S was very weakly active. All the remaining substitutions, including H20K, H20R, H20E, and H20D, the obvious candidates to mimic a protonated state or support proton translocation, were inactive. A second-site suppressor, ExbB(A39E), indiscriminately reactivated the majority of H20 substitutions and relocations, including H20V, which cannot be made protonatable. These results suggested that the TonB TMD was not on a proton conductance pathway and thus only indirectly responds to PMF, probably via ExbD.

The TonB dimeric crystal structures do not exist in vivo

The TonB system energizes transport of nutrients across the outer membrane of Escherichia coli using cytoplasmic membrane proton motive force (PMF) for energy. Integral cytoplasmic membrane proteins ExbB and ExbD appear to harvest PMF and transduce it to TonB. The carboxy terminus of TonB then physically interacts with outer membrane transporters to allow translocation of ligands into the periplasmic space. The structure of the TonB carboxy terminus (residues ~150 to 239) has been solved several times with similar results. Our previous results hinted that in vitro structures might not mimic the dimeric conformations that characterize TonB in vivo. To test structural predictions and to identify irreplaceable residues, the entire carboxy terminus of TonB was scanned with Cys substitutions. TonB I232C and N233C, predicted to efficiently form disulfide-linked dimers in the crystal structures, did not do so. In contrast, Cys substitutions positioned at large distances from one another in the crystal structures efficiently formed dimers. Cys scanning identified seven functionally important residues. However, no single residue was irreplaceable. The phenotypes conferred by changes of the seven residues depended on both the specific assay used and the residue substituted. All seven residues were synergistic with one another. The buried nature of the residues in the structures was also inconsistent with these properties. Taken together, these results indicate that the solved dimeric crystal structures of TonB do not exist. The most likely explanation for the aberrant structures is that they were obtained in the absence of the TonB transmembrane domain, ExbB, ExbD, and/or the PMF.

The TonB system of Gram-negative bacteria is an attractive target for development of novel antibiotics because of its importance in iron acquisition and virulence. Logically, therefore, the structure of TonB must be accurately understood. TonB functions as a dimer in vivo, and two different but similar crystal structures of the dimeric carboxy-terminal ~90 amino acids gave rise to mechanistic models. Here we demonstrate that the crystal structures, and therefore the models based on them, are not biologically relevant. The idiosyncratic phenotypes conferred by substitutions at the only seven functionally important residues in the carboxy terminus suggest that similar to interaction of cytochromes P450 with numerous substrates, these residues allow TonB to differentially interact with different outer membrane transporters. Taken together, data suggest that TonB is maintained poised between order and disorder by ExbB, ExbD, and the proton motive force (PMF) before energy transduction to the outer membrane transporters.

Cytoplasmic membrane protonmotive force energizes periplasmic interactions between ExbD and TonB

The TonB system of Escherichia coli (TonB/ExbB/ExbD) transduces the protonmotive force (pmf) of the cytoplasmic membrane to drive active transport by high-affinity outer membrane transporters. In this study, chromosomally encoded ExbD formed formaldehyde-linked complexes with TonB, ExbB and itself (homodimers) in vivo. Pmf was required for detectable cross-linking between TonB-ExbD periplasmic domains. Consistent with that observation, the presence of inactivating transmembrane domain mutations ExbD(D25N) or TonB(H20A) also prevented efficient formaldehyde cross-linking between ExbD and TonB. A specific site of periplasmic interaction occurred between ExbD(A92C) and TonB(A150C) and required functional transmembrane domains in both proteins. Conversely, neither TonB, ExbB nor pmf were required for ExbD dimer formation. These data suggest two possible models where either dynamic complex formation occurred through transmembrane domains or the transmembrane domains of ExbD and TonB configure their respective periplasmic domains. Analysis of T7-tagged ExbD with anti-ExbD antibodies revealed that a T7 tag was responsible both for our previous failure to detect T7-ExbD-ExbB and T7-ExbD-TonB formaldehyde-linked complexes and for the concomitant artefactual appearance of T7-ExbD trimers.

TonB system, in vivo assays and characterization

The multiprotein TonB system of Escherichia coli involves proteins in both the cytoplasmic membrane and the outer membrane. By a still unclear mechanism, the proton-motive force of the cytoplasmic membrane is used to catalyze active transport through high-affinity transporters in the outer membrane. TonB, ExbB, and ExbD are required to transduce the cytoplasmic membrane energy to these transporters. For E. coli, transport ligands consist of iron-siderophore complexes, vitamin B(12), group B colicins, and bacteriophages T1 and ø80. Our experimental philosophy is that data gathered in vivo, where all known and unknown components are present at balanced chromosomal levels in the whole cell, can be interpreted with less ambiguity than when a subset of components is overexpressed or analysed in vitro. This chapter describes in vivo assays for the TonB system and their application.

Colicin biology

Colicins are proteins produced by and toxic for some strains of Escherichia coli. They are produced by strains of E. coli carrying a colicinogenic plasmid that bears the genetic determinants for colicin synthesis, immunity, and release. Insights gained into each fundamental aspect of their biology are presented: their synthesis, which is under SOS regulation; their release into the extracellular medium, which involves the colicin lysis protein; and their uptake mechanisms and modes of action. Colicins are organized into three domains, each one involved in a different step of the process of killing sensitive bacteria. The structures of some colicins are known at the atomic level and are discussed. Colicins exert their lethal action by first binding to specific receptors, which are outer membrane proteins used for the entry of specific nutrients. They are then translocated through the outer membrane and transit through the periplasm by either the Tol or the TonB system. The components of each system are known, and their implication in the functioning of the system is described. Colicins then reach their lethal target and act either by forming a voltage-dependent channel into the inner membrane or by using their endonuclease activity on DNA, rRNA, or tRNA. The mechanisms of inhibition by specific and cognate immunity proteins are presented. Finally, the use of colicins as laboratory or biotechnological tools and their mode of evolution are discussed.

His(20) provides the sole functionally significant side chain in the essential TonB transmembrane domain

The cytoplasmic membrane protein TonB couples the protonmotive force of the cytoplasmic membrane to active transport across the outer membrane of Escherichia coli. The uncleaved amino-terminal signal anchor transmembrane domain (TMD; residues 12 to 32) of TonB and the integral cytoplasmic membrane proteins ExbB and ExbD are essential to this process, with important interactions occurring among the several TMDs of all three proteins. Here, we show that, of all the residues in the TonB TMD, only His(20) is essential for TonB activity. When alanyl residues replaced all TMD residues except Ser(16) and His(20), the resultant "all-Ala Ser(16) His(20)" TMD TonB retained 90% of wild-type iron transport activity. Ser(16)Ala in the context of a wild-type TonB TMD was fully active. In contrast, His(20)Ala in the wild-type TMD was entirely inactive. In more mechanistically informative assays, the all-Ala Ser(16) His(20) TMD TonB unexpectedly failed to support formation of disulfide-linked dimers by TonB derivatives bearing Cys substitutions for the aromatic residues in the carboxy terminus. We hypothesize that, because ExbB/D apparently cannot efficiently down-regulate conformational changes at the TonB carboxy terminus through the all-Ala Ser(16) His(20) TMD, the TonB carboxy terminus might fold so rapidly that disulfide-linked dimers cannot be efficiently trapped. In formaldehyde cross-linking experiments, the all-Ala Ser(16) His(20) TMD also supported large numbers of apparently nonspecific contacts with unknown proteins. The all-Ala Ser(16) His(20) TMD TonB retained its dependence on ExbB/D. Together, these results suggest that a role for ExbB/D might be to control rapid and nonspecific folding that the unregulated TonB carboxy terminus otherwise undergoes. Such a model helps to reconcile the crystal/nuclear magnetic resonance structures of the TonB carboxy terminus with conformational changes and mutant phenotypes observed at the TonB carboxy terminus in vivo.

TonB-dependent energy transduction between outer and cytoplasmic membranes

The TonB system of Escherichia coli (and most other Gram-negative bacteria) is distinguished by its importance to iron acquisition, its contribution to bacterial pathogenesis, and a unique and mysterious mechanism of action. This system somehow gathers the potential energy of the cytoplasmic membrane (CM) proton gradient and delivers it to active transporters in the outer membrane (OM). Our understanding of this system is confounded by the challenge of reconciling often contradictory in vivo and in vitro studies that are presented in this review.

Disulphide trapping of an in vivo energy-dependent conformation of Escherichia coli TonB protein

In Escherichia coli, the TonB system transduces the protonmotive force (pmf) of the cytoplasmic membrane to support a variety of transport events across the outer membrane. Cytoplasmic membrane proteins ExbB and ExbD appear to harvest pmf and transduce it to TonB. Experimental evidence suggests that TonB shuttles to the outer membrane, apparently to deliver conformationally stored potential energy to outer membrane transporters. In the most recent model, discharged TonB is then recycled to the cytoplasmic membrane to be re-energized by the energy coupling proteins, ExbB/D. It has been suggested that the carboxy-terminal 75 amino acids of active TonB could be represented by the rigid, strand-exchanged, dimeric crystal structure of the corresponding fragment. In contrast, recent genetic studies of alanine substitutions have suggested instead that in vivo the carboxy-terminus of intact TonB is dynamic and flexible. The biochemical studies presented here confirm and extend those results by demonstrating that individual cys substitution at aromatic residues in one monomeric subunit can form spontaneous dimers in vivo with the identical residue in the other monomeric subunit. Two energized TonBs appear to form a single cluster of 8-10 aromatic amino acids, including those found at opposite ends of the crystal structure. The aromatic cluster requires both the amino-terminal energy coupling domain of TonB, and ExbB/D (and cross-talk analogues TolQ/R) for in vivo formation. The large aromatic cluster is detected in cytoplasmic membrane-, but not outer membrane-associated TonB. Consistent with those observations, the aromatic cluster can form in the first half of the energy transduction cycle, before release of conformationally stored potential energy to ligand-loaded outer membrane transporters. The model that emerges is one in which, after input of pmf mediated through ExbB/D and the TonB transmembrane domain, the TonB carboxy-terminus can form a meta-stable high-energy conformation that is not represented by the crystal structure of the carboxy-terminus.

TonB interacts with nonreceptor proteins in the outer membrane of Escherichia coli

The Escherichia coli TonB protein serves to couple the cytoplasmic membrane proton motive force to active transport of iron-siderophore complexes and vitamin B(12) across the outer membrane. Consistent with this role, TonB has been demonstrated to participate in strong interactions with both the cytoplasmic and outer membranes. The cytoplasmic membrane determinants for that interaction have been previously characterized in some detail. Here we begin to examine the nature of TonB interactions with the outer membrane. Although the presence of the siderophore enterochelin (also known as enterobactin) greatly enhanced detectable cross-linking between TonB and the outer membrane receptor, FepA, the absence of enterochelin did not prevent the localization of TonB to the outer membrane. Furthermore, the absence of FepA or indeed of all the iron-responsive outer membrane receptors did not alter this association of TonB with the outer membrane. This suggested that TonB interactions with the outer membrane were not limited to the TonB-dependent outer membrane receptors. Hydrolysis of the murein layer with lysozyme did not alter the distribution of TonB, suggesting that peptidoglycan was not responsible for the outer membrane association of TonB. Conversely, the interaction of TonB with the outer membrane was disrupted by the addition of 4 M NaCl, suggesting that these interactions were proteinaceous. Subsequently, two additional contacts of TonB with the outer membrane proteins Lpp and, putatively, OmpA were identified by in vivo cross-linking. These contacts corresponded to the 43-kDa and part of the 77-kDa TonB-specific complexes described previously. Surprisingly, mutations in these proteins individually did not appear to affect TonB phenotypes. These results suggest that there may be multiple redundant sites where TonB can interact with the outer membrane prior to transducing energy to the outer membrane receptors.

Interactions in the TonB-dependent energy transduction complex: ExbB and ExbD form homomultimers

The cytoplasmic membrane proteins ExbB and ExbD support TonB-dependent active transport of iron siderophores and vitamin B12 across the essentially unenergized outer membrane of Escherichia coli. In this study, in vivo formaldehyde cross-linking analysis was used to investigate the interactions of T7 epitope-tagged ExbB or ExbD proteins. ExbB and ExbD each formed two unique cross-linked complexes which were not dependent on the presence of TonB, the outer membrane receptor protein FepA, or the other Exb protein. Cross-linking analysis of ExbB- and ExbD-derived size variants demonstrated instead that these ExbB and ExbD complexes were homodimers and homotrimers and suggested that ExbB also interacted with an unidentified protein(s). Cross-linking analysis of epitope-tagged ExbB and ExbD proteins with TonB antisera afforded detection of a previously unrecognized TonB-ExbD cross-linked complex and confirmed the composition of the TonB-ExbB cross-linked complex. The implications of these findings for the mechanism of TonB-dependent energy transduction are discussed.

Vibrio cholerae iron transport: haem transport genes are linked to one of two sets of tonB, exbB, exbD genes

Vibrio cholerae was found to have two sets of genes encoding TonB, ExbB and ExbD proteins. The first set (tonB1, exbB1, exbD1) was obtained by complementation of a V. cholerae tonB mutant. In the mutant, a plasmid containing these genes permitted transport via the known V. cholerae high-affinity iron transport systems, including uptake of haem, vibriobactin and ferrichrome. When chromosomal mutations in exbB1 or exbD1 were introduced into a wild-type V. cholerae background, no defect in iron transport was noted, indicating the existence of additional genes that can complement the defect in the wild-type background. Another region of the V. cholerae chromosome was cloned that encoded a second functional TonB/Exb system (tonB2, exbB2, exbD2). A chromosomal mutation in exbB2 also failed to exhibit a defect in iron transport, but a V. cholerae strain that had chromosomal mutations in both the exbB1 and exbB2 genes displayed a mutant phenotype similar to that of an Escherichia coli tonB mutant. The genes encoding TonB1, ExbB1, ExbD1 were part of an operon that included three haem transport genes (hutBCD), and all six genes appeared to be expressed from a single Fur-regulated promoter upstream of tonB1. A plasmid containing all six genes permitted utilization of haem by an E. coli strain expressing the V. cholerae haem receptor, HutA. Analysis of the hut genes indicated that hutBCD, which are predicted to encode a periplasmic binding protein (HutB) and cytoplasmic membrane permease (HutC and HutD), were required to reconstitute the V. cholerae haem transport system in E. coli. In V. cholerae, the presence of hutBCD stimulated growth when haemin was the iron source, but these genes were not essential for haemin utilization in V. cholerae.

Fitness consequences of genetically engineered herbicide and antibiotic resistance in Arabidopsis thaliana

Researchers have often invoked the concept of metabolic drain to explain the lower growth rates of bacteria containing plasmids that confer antibiotic resistance. This idea posits that the energetic input needed to produce detoxifying enzymes diverts resources from clonal reproduction. In this paper we examine whether the concept of metabolic drain can be applied successfully to plants that differ from bacteria in several key aspects including their relative genome size and reproductive rate. We have conducted a field experiment using mutant and transgenic Arabidopsis thaliana that allows the comparison of genotypes differing by a single gene conferring resistance to either the herbicide chlorsulfuron or the antibiotic kanamycin. In addition to testing whether these traits reduce fitness, this experiment was conducted at two levels of resource availability to examine whether costs of resistance are sensitive to environmental quality. We found that herbicide-resistant individuals produced 26% fewer seeds than susceptible counterparts. However, contrasting published results in bacterial systems, the fecundity of individuals was completely unaffected by the expression of an introduced antibiotic resistance gene. The fitness cost associated with chlorsulfuron resistance was greater in nutrient-poor conditions relative to nutrient-rich conditions for comparisons involving mutant, but not transgenic, genotypes.

Identification of TonB homologs in the family Enterobacteriaceae and evidence for conservation of TonB-dependent energy transduction complexes

The transport of Fe(III)-siderophore complexes and vitamin B12 across the outer membrane of Escherichia coli requires the TonB-dependent energy transduction system. A set of murine monoclonal antibodies (MAbs) was generated against an E. coli TrpC-TonB fusion protein to facilitate structure and function studies. In the present study, the epitopes recognized by these MAbs were mapped, and their distribution in gram-negative organisms was examined. Cross-species reactivity patterns obtained against TonB homologs of known sequence were used to refine epitope mapping, with some epitopes ultimately confirmed by inhibition experiments using synthetic polypeptides. Epitopes recognized by this set of MAbs were conserved in TonB homologs for 9 of 12 species in the family Enterobacteriaceae (including E. coli), including previously unidentified TonB homologs in Shigella, Citrobacter, Proteus, and Kluyvera species. These homologs were also detected by a polyclonal alpha-TrpC-TonB serum that additionally recognized the known Yersinia enterocolitica TonB homolog and a putative TonB homolog in Edwardsiella tarda. These antibody preparations failed to detect the known TonB homologs of either Pseudomonas putida or Haemophilus influenzae but did identify potential TonB homologs in several other nonenteric gram-negative species. In vivo chemical cross-linking experiments demonstrated that in addition to TonB, auxiliary components of the TonB-dependent energy transduction system are broadly conserved in members of the family Enterobacteriaceae, suggesting that the TonB system represents a common system for high-affinity active transport across the gram-negative outer membrane.

Metabolic load and heterologous gene expression

The expression of a foreign protein(s) in a recombinant host cell or organism often utilizes a significant amount of the host cell's resources, removing those resources away from host cell metabolism and placing a metabolic load (metabolic drain, metabolic burden) on the host. As a consequence of the imposed metabolic load, the biochemistry and physiology of the host may be dramatically altered. The numerous physiological changes that may occur often lowers the amount of the target foreign protein that is produced and eventually recovered from the recombinant organism. In this review the physiological changes to host cells, the causes of the phenomenon of metabolic load, and several strategies to avoid some of the problems associated with metabolic load are elaborated and discussed.

Bacterial aspects associated with the expression of a single-chain antibody fragment in Escherichia coli

The bacterial expression of a single-chain antibody fragment, designated L6 sFv, was examined. Periplasmic targeting resulted in the production of a correctly folded protein that bound tumor antigen. However, immediately after induction at either 30 degrees C or 37 degrees C there was a significant loss in bacterial viability, which was followed by a loss in absorbance. The loss in absorbance correlated with cell lysis and release of the L6 sFv into the culture supernatant. The kinetics of appearance of L6 sFv in the supernatant paralleled that of periplasmic beta-lactamase and confirmed an initial loss of cell-wall integrity prior to cell lysis. Bacteria incubated at 30 degrees C produced approximately threefold more correctly folded antibody fragment because of an increase in the number of cells/A660 at the lower incubation temperature. More than 95% of the L6 sFv, made at either incubation temperature, was incorrectly folded. Osmotic-shock procedures did not release L6 sFv. However, in situ subtilisin susceptibility experiments with bacterial spheroplasts confirmed a periplasmic location. French press disruption resulted in the release of correctly but not incorrectly folded material. Membrane fractionation revealed that the incorrectly folded L6 sFv remained associated with both the inner and outer membrane. These results demonstrate that, in this system, antibody fragment expression resulted initially in cell death, which was followed by release of protein into the culture supernatant and eventually cell lysis. It is also suggested that membrane association in the periplasmic space may impede proper folding.

Partial suppression of an Escherichia coli TonB transmembrane domain mutation (delta V17) by a missense mutation in ExbB

Active transport of vitamin B12 and Fe(III)-siderophore complexes across the outer membrane of Escherichia coli appears to be dependent upon the ability of the TonB protein to couple cytoplasmic membrane-generated protonmotive force to outer membrane receptors. TonB is supported in this role by an auxiliary protein, ExbB, which, in addition to stabilizing TonB against the activities of endogenous envelope proteases, directly contributes to the energy transduction process. The topological partitioning of TonB and ExbB to either side of the cytoplasmic membrane restricts the sites of interaction between these proteins primarily to their transmembrane domains. In this study, deletion of valine 17 within the aminoterminal transmembrane anchor of TonB resulted in complete loss of TonB activity, as well as loss of detectable in vivo crosslinking into a 59 kDa complex believed to contain ExbB. The delta V17 mutation had no effect on TonB export. The loss of crosslinking appeared to reflect conformational changes in the TonB/ExbB pair rather than loss of interaction since ExbB was still required for some stabilization of TonB delta V17. Molecular modeling suggested that the delta V17 mutation caused a significant change in the predicted conserved face of the TonB amino-terminal membrane anchor. TonB delta V17 was unable to achieve the 23 kDa proteinase K-resistant form in lysed sphaeroplasts that is characteristic of active TonB. Wild-type TonB also failed to achieve the proteinase K-resistant configuration when ExbB was absent. Taken together these results suggested that the delta V17 mutation interrupted productive TonB-ExbB interactions. The apparent ability to crosslink to ExbB as well as a limited ability to transduce energy were restored by a second mutation (A39E) in or near the first predicted transmembrane domain of the ExbB protein. Consistent with the weak suppression, a 23 kDa proteinase K-resistant form of TonB delta V17 was not observed in the presence of ExbBA39E. Neither the ExbBA39E allele nor the absence of ExbB affected TonB or TonB delta V17 export. Unlike the tonB delta V17 mutation, the exbBA39E mutation did not greatly alter a modelled ExbB transmembrane domain structure. Furthermore, the suppressor ExbBA39E functioned normally with wild-type TonB, suggesting that the suppressor was not allele specific. Contrary to expectations, the TonB delta V17, ExbBA39E pair resulted in a TonB with a greatly reduced half-life (approximately 10 min). These results together with protease susceptibility studies suggest that ExbB functions by modulating the conformation of TonB.

Role of the TonB amino terminus in energy transduction between membranes

Escherichia coli TonB protein is an energy transducer, coupling cytoplasmic membrane energy to active transport of vitamin B12 and iron-siderophores across the outer membrane. TonB is anchored in the cytoplasmic membrane by its hydrophobic amino terminus, with the remainder occupying the periplasmic space. In this report we establish several functions for the hydrophobic amino terminus of TonB. A G-26-->D substitution in the amino terminus prevents export of TonB, suggesting that the amino terminus contains an export signal for proper localization of TonB within the cell envelope. Substitution of the first membrane-spanning domain of the cytoplasmic membrane protein TetA for the TonB amino terminus eliminates TonB activity without altering TonB export, suggesting that the amino terminus contains sequence-specific information. Detectable TonB cross-linking to ExbB is also prevented, suggesting that the two proteins interact primarily through their transmembrane domains. In vivo cleavage of the amino terminus of TonB carrying an engineered leader peptidase cleavage site eliminates (i) TonB activity, (ii) detectable interaction with a membrane fraction having a density intermediate to those of the cytoplasmic and outer membranes, and (iii) cross-linking to ExbB. In contrast, the amino terminus is not required for cross-linking to other proteins with which TonB can form complexes, including FepA. Additionally, although the amino terminus clearly is a membrane anchor, it is not the only means by which TonB associates with the cytoplasmic membrane. TonB lacking its amino-terminal membrane anchor still remains largely associated with the cytoplasmic membrane.

TonB protein and energy transduction between membranes

TonB protein couples cytoplasmic membrane electrochemical potential to active transport of iron-siderophore complexes and vitamin B12 through high-affinity outer membrane receptors of Gram-negative bacteria. The mechanism of energy transduction remains to be determined, but important concepts have already begun to emerge. Consistent with its function, TonB is anchored in the cytoplasmic membrane by its uncleaved amino terminus while largely occupying the periplasm. Both the connection to the cytoplasmic membrane and the amino acid sequences of the anchor are essential for activity. TonB directly associates with a number of envelope proteins, among them the outer membrane receptors and cytoplasmic membrane protein ExbB. ExbB and TonB interact through their respective transmembrane domains. ExbB is proposed to recycle TonB to an active conformation following energy transduction to the outer membrane. TonB most likely associates with the outer membrane receptors through its carboxy terminus, which is required for function. In contrast, the novel proline-rich region of TonB can be deleted without affecting function. A model that incorporates this information, as well as tempered speculation, is presented.

Molecular characterization of the Enterobacter aerogenes tonB gene: identification of a novel type of tonB box suppressor mutant

The tonB gene of Enterobacter aerogenes was cloned, sequenced, and expressed in Escherichia coli. It complemented an E. coli tonB mutant as efficiently as E. coli tonB, except for colicin B and D sensitivities. However, colicin B and D sensitivities were complemented by a derivative in which the aspartate at position 165 was replaced by a glutamine (TonBD-165-->Q) by site-directed mutagenesis. In E. coli, the corresponding amino acid is a glutamine (Q-160) which is known to be altered in most mutants showing suppression of the btuB451 mutation. Fourteen independent btuB451 suppressor mutations in E. aerogenes tonB which all had suffered the same point mutation resulting in a change from glycine to valine at position 239 (G-239-->V) of the C-terminal end of the protein were isolated. The mutation was located within a region which is nonessential for function of E. aerogenes TonB as well as E. coli TonB. A constructed double mutation, expressing a D-165-->Q/G-239-->V derivative, no longer acted as a btuB451 suppressor. However, it restored colicin B and D sensitivities even more efficiently than the D-165-->Q derivative. Corresponding mutations constructed in E. coli tonB, giving rise to Q-160-->D, G-234-->V, and Q-160-->D/G-234-->V derivatives, showed phenotypes comparable to the E. aerogenes mutations. We take this as evidence that at least a functional interaction between the D-165 (Q-160 in E. coli) and the G-239 (G-234 in E. coli) region is necessary for TonB function. The implications of this interaction for functional instability of TonB are discussed.

Cloning and sequencing of the Klebsiella pneumoniae tonB gene and characterization of Escherichia coli-K. pneumoniae TonB hybrid proteins

The tonB gene of Klebsiella pneumoniae was cloned on a 1.4-kb EcoRV fragment and expressed in Escherichia coli. Determination of the nucleotide (nt) sequence of the cloned fragment identified tonB as an open reading frame (ORF) of 729 nt. The TonB protein consists of 243 amino acids (aa) and is highly homologous to Enterobacter aerogenes TonB. The genetic context of K. pneumoniae and En. aerogenes tonB is the same, but is different from that of E. coli, Salmonella typhimurium and Serratia marcescens. K. pneumoniae tonB complemented an E. coli tonB mutant as efficiently as E. coli tonB when cloned on the same type of plasmid. Replacement of the highly hydrophobic C terminus of E. coli TonB by the weakly hydrophobic C terminus of K. pneumoniae TonB yielded a hybrid protein of the same activity as E. coli wild-type TonB. The result shows that the strong hydrophobicity of the C-terminal end is not necessary for the function of E. coli TonB.

Evidence for a TonB-dependent energy transduction complex in Escherichia coli

Escherichia coli TonB protein is required for the active transport of vitamin B12 and Fe(III)-siderophore complexes across the outer membrane, infection by bacteriophages T1 and phi 80, and sensitivity to B-group colicins. TonB appears to function as an energy transducer in these processes, coupling cytoplasmic membrane electrochemical potential to receptors in the outer membrane. Previous reports have demonstrated that chromosomally encoded TonB is functionally unstable in the absence of protein synthesis (half-life approximately 15-30 minutes) and have shown that plasmid-encoded, overexpressed TonB is chemically unstable (half-life approximately 5 minutes). In contrast, this study has shown that chromosomally encoded TonB was chemically stable for greater than 90 minutes while maintaining its functional instability. These data suggest that proteolytic degradation of TonB protein is not the basis of its functional instability. Auxiliary proteins such as ExbB also play a role in TonB-dependent energy transduction. In this study, we have shown that the chemical half-life of chromosomally encoded TonB in an exbB::Tn10 mutant was reduced at least 18-fold, suggesting that TonB is a part of a cytoplasmic membrane complex that includes, at the minimum, ExbB.(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of Escherichia coli TonB membrane topology by use of PhoA fusions

Alkaline phosphatase (PhoA) fusions to TonB amino acids 32, 60, 125, 207, and 239 (the carboxy terminus) all showed high PhoA activity; a PhoA fusion to TonB amino acid 12 was inactive. The full-length TonB-PhoA fusion protein was associated with the cytoplasmic membrane and retained partial TonB function. These results support a model in which TonB is anchored in the cytoplasmic membrane by its hydrophobic amino terminus, with the remainder of the protein, including its hydrophobic carboxy terminus, extending into the periplasm.

Functional analysis of a C-terminally altered TonB protein of Escherichia coli

Deletions within the 3'-end of the cloned Escherichia coli tonB gene were constructed and recombined into the chromosome. Deletions affecting the last eight C-terminal amino acids (aa) yielded functionally active TonB, whereas deletion of the last 15 C-terminal aa, which removed the C-terminal hydrophobic region of TonB, abolished TonB function entirely. Distribution of TonB within cell compartments was not significantly affected by any of the deletions. All deletion derivatives were less stable than wild-type TonB. A major degradation product of approx. 29 kDa was observed for all TonB derivatives, indicating that it was an N-terminal fragment. The possibility is discussed that the C-terminal hydrophobic region is involved in the intramolecular interaction rather than in the membrane association of TonB.

TonB and the gram-negative dilemma

TonB protein serves as an energy transducer to couple cytoplasmic membrane energy to high-affinity active transport of iron siderophores and vitamin B12 across the outer membranes of Gram-negative bacteria. The biochemical mechanism of the energy transduction remains to be determined, but important details are already known. TonB is targeted to and anchored in the cytoplasmic membrane by a single membrane-spanning domain and spans the periplasm to physically interact with outer-membrane receptors of the transport ligands. TonB-dependent energy transduction is modulated by ExbB protein, which stabilizes TonB, and possibly by several other proteins including ExbC, ExbD, and TolQ. TonB has a relatively short functional half-life that is accelerated when rates of active transport across the outer membrane are increased. A model that incorporates this information, as well as some tempered speculation, is presented.

Aerobic regulation of the Escherichia coli tonB gene by changes in iron availability and the fur locus

The tonB gene is required for the transport of several different iron-siderophore complexes across the Escherichia coli outer membrane. In this study, transcriptional regulation of the tonB gene was investigated by using three different tonB-lacZ fusions to monitor tonB expression under aerobic conditions and in the presence of a wild-type tonB gene. Prior work by other laboratories suggests that tonB is expressed at low constitutive levels regardless of changes in iron availability or the fur locus. In contrast, these data show that tonB transcription is repressed threefold by growth in the presence of FeCl3 compared with growth in the presence of the iron chelator dipyridyl and that this repression requires the fur locus. A 168-base-pair DNA fragment carrying the tonB promoter was sufficient for the observed transcriptional regulation. In addition, the tonB gene appeared to have a substantially stronger promoter than previously recognized. The inability of other laboratories to detect tonB transcription regulation appears to be due to the extremely slow growth of iron-starved tonB strains and the use of Mu d1(lac Apr)- or lambda plac Mu53-generated fusions that encode a thermolabile TrpA-LacZ hybrid protein. The data also suggest that the previously reported growth phase regulation of tonB occurs only in media with intermediate levels of available iron and is due to iron starvation-induced derepression as the culture approaches stationary phase.